The family of NF-kB transcription factors together with their associated regulators, the family of IkB proteins, are critical host defense mechanisms; they control the expression of many immune effectors as well as proteins that counteract stress. NF-kB factors are also required for the expression of HIV. Although various family members appear to overlap functionally, they must also have unique functions. An understanding of the unique functions may offer more specific and limited targets for immunomodulatory therapies in select diseases where a complete block of all NF-kB activity is undesirable. We are investigating the unique biologic roles of NF-kB and IkB family members with so-called 'knock-out' mice; we have generated mice deficient in expression of p52, a subunit of NF-kB, and we have generated mice deficient in both p52 and its most highly related family member, p50, in order to reveal functions shared by these two proteins. Previously, we have generated and analyzed mice deficient in Bcl-3, an IkB family member. Both Bcl-3 and p52 are potential oncoproteins as their respective genes have been shown to be associated with recurrent chromosomal translocations in certain lymphocytic leukemias. To uncover the physiologic roles of these proteins we have challenged the mutant mice with antigens, viruses and pathogens to reveal impaired immune responses. Defects in p52 knock-out mice resemble those seen in Bcl-3 knock-out mice and include impaired production of antibodies with switched isotypes, absence of germinal center reactions, and disruption of proper microarchitecture in both spleens and lymph nodes, including disruption of the splenic marginal zone, loss of B cell follicles and apparent loss of follicular dendritic cells. The similarity in phenotypes is consistent with the notion previously established in this laboratory that p52 homodimers and Bcl-3 together can form transactivating complexes. Adoptive transfers into Rag-deficient mice have revealed that p52 or Bcl-3 deficient lymphocytes are capable of forming germinal centers. Thus, the loss of germinal centers in the knock-outs is not due to defects in lymphocytes but instead may be due to defects in follicular dendritic cells (FDC). Loss of FDCs may also underlie the loss of B cell follicles. In contrast to p52 or Bcl-3 or p50 single knock-out mice, the p50/p52 double knock-out mice have several severe defects seen even in the absence of immune challenge and they die a few weeks after birth, in part due to severe osteopetrosis. Osteopetrosis is a consequence of impaired development of the hematopoietic stem cell-derived osteoclasts, an unexpected role for NF-kB factors. Adoptive transfer of wild-type bone marrow into double knock-out animals repairs the osteoclast defect. p50/p52 double knock-out animals also have severe disruptions of splenic and thymic microarchitecture, including apparent loss of medullary thymic epithelial cells, peripheral T cells and mature B cells. Adoptive transfers of double knock-out bone marrow into Rag-deficient mice reveals a block in the development of mutant immature to mature B cells, an unanticipated finding. These data open up new insights into the role of NF-kB in cellular development and a new target for the prevention of osteoporosis.